1. Field of the Invention
The present invention relates to an optical recording medium that is suitable for making a record with laser beams, particularly semiconductor laser beams. More particularly, it relates to an optical recording medium that can be used for optical disc or optical card techniques.
2. Related Background Art
Generally speaking, optical recording mediums such as optical discs or optical cards can record information at a high density by forming small pits of, for example, about 1 .mu.m which can be optically detected on a thin recording layer provided on a substrate, having spiral, circular or linear grooves.
When a laser beam converged on the surface of a recording layer is scanned, the recording layer absorbs laser energy to form optically detectable pits, whereby the information is written.
For example, according to a heat mode recording system, the recording layer absorbs heat energy and small concave pits can be thereby formed by evaporation or melting at that site. According to another heat mode recording system, as a result of the absorption of the laser energy applied, pits having optically detectable density difference can be formed at that site.
Recording mediums having been proposed as the optical recording mediums used in such optical discs and optical cards include those mainly using metallic thin films such as an aluminum-deposited film, and inorganic materials such as a bismuth thin film, a tellurium oxide film and a chalcogenide amorphous glass film. These thin films commonly show a sensitivity to the light with wavelengths of about 350 to 800 nm and also have a high reflectance to laser beams. Thus they have the disadvantages such that the laser beams are utilized at a low rate.
Under such circumstances, researches have been made on an optical recording medium comprising a thin film of an organic compound that can cause optical changes of physical properties, using light energy of relatively long wavelengths (for example, 780 nm or more). Such an optical recording medium can form the pits, i.e., holes, concaves, and/or optical changes, using a semiconductor laser having an oscillation wavelength of about 780 nm or about 830 nm and also the thin film of an organic compound can be formed by coating, so that optical recording mediums can be obtained at a low cost.
In general, however, the organic compound having absorption characteristics at the long wavelength side has the problem that it is unstable to heat.
For example, optical recording mediums are not always handled only in offices provided with satisfactory air-conditioning equipment, and it must be taken into account that they are stored in warehouses for transportation or suffer rise of internal temperature in recording-reproducing units. Thus, it has been sought to provide a medium having better stability in high-temperature atmospheres. In particular, when the optical recording medium is an optical card, it is difficult for the medium to take hollow structure (or air-sandwich structure) from the viewpoint of the thickness and strength required for the card. Hence, it necessarily takes laminated structure in which an opposed substrate is directly laminated on a recording layer with an adhesive. The adhesive effectively usable for such purpose includes hot-melt adhesives, which may not lower the sensitivity for the writing in the recording layer and have excellent workability. When, however, the hot-melt adhesives are used, the temperature reaches about 100.degree. C. at the time of lamination though for a short period of time. Thus, it has been sought to provide a medium comprising a recording layer having better high-temperature resistance.
Moreover, in many instances, optical cards are handled while they are personally carried and there is a high possibility that they are handled carelessly. Thus, it has been sought to provide a medium having excellent environmental stability including the high-temperature resistance.
As the optical recording medium of this type, mediums with a variety of constitution have been proposed. For example, Japanese Laid-Open Patent Application No. 62-132681 discloses an optical recording medium having good light-resistance and resistance to reproducing light, which comprises a recording layer containing a naphtholactam compound as represented by the following formula: ##STR1## wherein K represents a substituted or unsubstituted aromatic amine residual group, R represents a substituted or unsubstituted alkyl group, cycloalkyl group, aryl group or ally group, and the ring A represents a substituted or unsubstituted naphthalene ring. Japanese Laid-Open Patent Application No. 62-230857 also discloses an optical recording medium containing a naphtholactam coloring matter as represented by the following formula, that can be stable and free from corrosion for a long period of time. ##STR2##